Hypertension is a condition characterized by prolonged periods of high blood pressure. Hypertension can lead to an enlarged or damaged heart (hypertrophy) and, eventually, heart failure. Though treatable, hypertension is the primary cause of death for tens of thousands of patients per year in the United States. Hypertension is also listed as a primary or contributing cause of death for hundreds of thousands of patients per year in the United States and affects an estimated 65 million people in the United Sates alone. Therefore, hypertension is a serious health problem necessitating significant research and development of effective treatment.
Blood pressure typically becomes elevated when resistance to blood flow increases. Increased resistance to blood flow can be caused by a variety of factors, including constriction of blood vessels and excessive fluid in the blood. For example, when blood vessels constrict due to plaque build-up on the lining of arterial walls, additional force is required to pump the same volume of blood through the blood vessels. Similarly, when fluid levels in the blood stream increase, additional force is required to pump blood throughout the body to meet the body's needs. The additional force required to maintain a sufficient volumetric flow rate of blood within a constricted space or in a diluted media increases blood pressure.
The body can generally tolerate short periods of increased blood pressure by activating a temporary autonomic response that causes blood pressure to decline. Specifically, the body's autonomic response inhibits the sympathetic nervous system and activates the parasympathetic nervous system. In inhibiting the sympathetic nervous system, the brain directs the heart to decrease cardiac output, the kidneys to reduce blood volume by expunging sodium and water, and the arterioles to dilate. In activating the parasympathetic nervous system, the brain relaxes the body's muscles, decreases the rate of respiration, and signals the heart to reduce the frequency of contractions. These physiologic changes can temporarily decrease blood pressure; however, they also produce other effects, such as fatigue and a reduced capacity for exercise.
When blood pressure becomes elevated, the body's autonomic response is triggered by stretch-sensitive mechanoreceptors, or baroreceptors, located in the walls of the heart and various major blood vessels. Rising blood pressure forces blood vessels to expand. This, in turn, causes baroreceptors located in vascular walls to become distended. As baroreceptors become distended, they generate action potentials more frequently, a physiologic process called baroreflex activation. The increased frequency of action potentials signals the brain to activate an autonomic response. In this manner, baroreceptors provide signals to the brain of changes in blood pressure.
The ability of baroreceptors to inform the brain of changes becomes compromised, however, as short-term changes in blood pressure become long-term changes. This is because baroreceptors detect changes in their distension with respect to a normalized state of distension that is constantly being recalibrated based upon a mean arterial pressure. The mean arterial pressure, called the set point, is typically in the range of 70-110 mmHg for normal, or healthy, blood pressure. If blood pressure remains elevated, such as due to hypertension, the set point will eventually recalibrate to a higher level. Similarly, if blood pressure remains low, such as due to hypotension, the set point will eventually calibrate to a lower level. As a result, hypertension desensitizes baroreceptors to high blood pressure, while hypotension desensitizes baroreceptors to low blood pressure. In the case of hypertension, the body's natural ability to lower blood pressure is thereby eroded as elevated blood pressure causes the set point to become normalized at a relatively high mean arterial pressure. As a result, baroreceptors are gradually prevented from initiating an autonomic response to lower dangerously high blood pressure.
If the volume of blood delivered through the vasculature becomes insufficient to meet the body's needs, such as due to arterial constriction or water absorption, the body may initiate a somatic response that increases blood flow by increasing cardiac output and heart rate. Although the rise in blood pressure that follows a somatic response normally poses littler threat, it can become dangerous if the ability to initiate an off-setting autonomic response to lower blood pressure has been compromised, such as due to hypertension. When blood pressure remains elevated following a somatic response, resulting myocardial damage to the heart may eventually reduce blood flow and result in progressively destructive somatic responses that leads to heart failure. It is therefore desirable to be able to initiate a reduction of blood pressure when hypertension has compromised the ability of baroreceptors to initiate such a response.
In an effort to treat hypertension, a number of different treatments have been developed. Some of these treatments focus on cause (high blood pressure), while others focus on effect (heart failure). For example, drug treatments have been proposed that reduce blood pressure. This form of treatment is often incompletely effective, however, since some patients may be unresponsive (refractory) to drug therapy. Drug therapy is also often accompanied by unwanted side effects and requires complex treatment regimens. These and other factors contribute to poor patient compliance with medical therapy. The development and administration of drug therapy is also expensive, adding to the high cost of health care already associated with these disorders.
Surgical procedures have also been proposed to treat hypertension. For example, heart transplantation has been proposed for patients who suffer from severe heart failure. Alternatively, a ventricular assist device may be implanted in the chest to increase the pumping action of the heart or an intra-aortic balloon pump may be used to maintain normal heart function for short periods of time. Cardiac resynchronization therapy may be also used to improve the coordination of contractions of the heart. Like drug treatment, however, surgical approaches are very costly. Surgical treatment is also associated with significant patient mortality rates. Moreover, surgical treatment often focuses on the effect of the problem (heart failure) rather than the source of the problem (hypertension), which fails to alter the natural history of the disease. Therefore, there is a need for improved treatments for hypertension.
One approach to treating the cause of hypertension is implanting electrodes capable of electrically stimulating nerves that produce an autonomic response, a function known as baropacing. For example, when the carotid sinus nerve is stimulated, the brain reduces or ceases activation of the sympathetic nervous system, thereby inhibiting the long-term cycle that can exacerbate heart failure. Electrical stimulation of the carotid sinus nerve (baropacing) has previously been proposed as a treatment for angina and high blood by decreasing the workload of the heart. U.S. Pat. No. 6,073,048 to Kieval, et al. discloses a baroreflex modulation system and method of baroreflex activation based on various cardiovascular and pulmonary parameters.
In treating hypertension, an alternative to nerve stimulation is stimulation of baroreceptors. This form of treatment is commonly known as baroreflex activation therapy, or “BAT.” In BAT, baroreflex activation can be achieved through an electro-stimulation of baroreceptors generated by implantable electrode assemblies, called baroreflex activation devices. Implantable electrode assemblies for electrotherapy or electro-stimulation are known in the art. U.S. Pat. No. 6,522,926 to Kieval, et al. discloses a baroreflex activation system and method for activating baroreceptors to regulate blood pressure. By treating hypertension through BAT, a coordinated electrical stimulation of baroreceptors produces the same physiologic response produced by baropacing while avoiding direct nerve stimulation.
Effecting a baroreflex response with a baroreflex activation device, such as through baropacing or baroreflex activation therapy, can be achieved by manual activation, by, for example, a physician. Since this form of treatment requires the presence of a physician who can monitor blood pressure at a given time and prescribe the necessary baropacing therapy, it is typically limited to short-term applications. To achieve effective long-term treatment of hypertension, baroreflex activation device capable of effecting a baroreflex response can be coordinated with a clinically implanted transducer that measures a physiologic parameter representative of blood pressure.
Such coordination with an implanted component presents a number of challenges. For example, because of differences in blood pressure between the high-pressure, arterial side of the vasculature and the low-pressure, venous side of the vasculature, it may be desirable to chronically measure blood pressure in the high-pressure side. Implantation into the high-pressure side, however, can involve additional health risks to a patient due to implantation of a medical device in the high-pressure blood vessels. Chronic implantation of leads with pressure sensors has generally been limited to the low pressure side where the medical risks associated with the long term viability of such implants are lower. While implantation of micro-transducers or micro-stimulators that could be used in connection with high-pressure blood vessels has been proposed, there has been a challenge in providing both effective power and communication to such miniature and isolated implants.
There remains a need for a chronically implanted medical device that integrates continuous measurements of a physiologic parameter, such as blood pressure, with selective modulation and/or regulation of blood pressure, as well a minimally-invasive procedure for implanting the device in the high-pressure side of the vasculature.